The invention relates to a method for providing mechanical and/or chemical treatment of water and/or waste water by way of sedimentation and thickening of the settleable solids contained therein. The invention further relates to apparatus to carry out this process. Methods for the treatment of water and/or waste water by mechanical and/or chemical means as a rule are characterized by separation of sedimentation and thickening. Both processes normally are subject to different determination of design rules.
As compared to thickening tanks which receive sludge from sedimentation basins, the latter usually require a surface area and volume 50 to 20 times larger than that of the thickening tanks. The basis for the determination of design of sedimentation stages is related to the requirements of effluent purity only. The necessary surface area depends on the specific settleability of the solids to be removed.
The term "thickening" refers exclusively to the process of maximum concentration of solids separated in the sedimentation phase in the sense of a preparatory step to economical, final dewatering and disposal. The most important part of the thickening process is based on fundamentally different factors as compared to sedimentation.
In general it may be assumed that in the mechanical and/or chemical treatment of water and/or waste water the sedimentation unit receives an amount of suspension per unit of time which is of a factor 20 to 100 times greater than that of the sludge volume introduced into the second stage thickener following the sedimentation stage. A thickening process as per the above definition, consequently, does not occur under conditions of sedimentation in conventional clarification units.
Both processes have in common that they are extraordinarily sensitive to hydraulic shock loads that always interfere with either sedimentation or thickening. Optimum process control cannot be attained without separating these two stages.
Depending on the specific requirements, the said processes may be combined with additional treatment steps, whereas in case of sedimentation the suspended, undissolved and/or colloidal pollutants are, in a suitable way, transformed into settleable matter.
Especially with respect to mechanical and chemical treatment of water and waste water, so-called sludge contact units are now being applied. These are specially designed for efficient sedimentation on the basis of the assumption that the settling speed of particles to be separated has to be increased for good efficiency. Suitable measures in this direction include addition of flocculants to the water and/or waste water as well as establishing the so-called sludge contact in which previously flocculated matter is mixed with raw water and flocculants in order to increase particle size of the solids in suspension.
Preparation of particles with a certain minimum settling speed is primarily required because of the fact that in the sedimentation zone of sludge contact units, like in other sedimentation systems, vertically or horizontally oriented flow patterns are being developed, which are more or less likely to interfere with the sedimentation of the flocks. Increasing flock size will, consequently, favor the settleability of pollutant solids and thereby increase the efficiency of the plant with respect to effluent quality.
Since the flocculation and sludge contact processes call for different turbulence conditions in the various treatment compartments of contact units; the conditions required for flocculation and destabilization may influence the sedimentation process. This is another reason why the thickening rate obtainable in sludge contact units in general is rather low. At the same time it is impossible to increase the hydraulic load on the sedimentation zone of a sludge contact plant to such an extent that the throughput relative to the effective surface space is higher than 2 to 5 m.sup.3 of liquid per m.sup.2 per hour. The sedimentation zone of a sludge contact plant requires in general 85% of the entire surface area of such systems.
If sedimentation and sludge removal are to be effective, a certain and minimum tank depth will be required. Therefore the load factor of conventional sedimentation tanks is kept relatively low as compared to the volume and surface space of the tank. Their degree of efficiency will not be optimized due to uncontrollable flow patterns forming inside the tank's settling zone. These uncontrollable sedimentation and flow conditions primarily account for the low solids concentration of separated sludge requiring sludge thickening prior to its further treatment. The sludge is usually thickened by mechanical means such as stirring devices in a separate thickening tank. Due to these reasons prior art apparatus of the described kind require much space while their initial cost is high and their efficiency not optimal.
In recent years, new high-efficiency sedimentation apparatus have been introduced comprising plates or tubes inclined relative to the horizontal plane and constituting a flow passage for the liquid to be separated. Their function is to shorten the settling paths. The sedimentation period is comparatively short, and the required space can be lowered.
Suggestions have been made for using such inclined plate or pipe separators for the sedimentation of activated sludges from biological treatment plants. However, the sludge settling in these bundled plates or pipes and sliding down their inner surfaces is not optimally thickened so that it requires some separate treatment too. In the case of counter-current apparatus of the described kind the sludge is therefore collected at the lower ends of the plates or tubes and taken by the way of conveying means to a separate thickener which it enters at definite point, so that it has to be distributed throughout the thickener. This leads to undesirable turbulences and dilution delaying the process; as a result, cost and space requirements increase.